better test documentation

src/test/run-pass/mir-inlining/no-trait-method-issue-40473.rs

@@ -17,6 +17,8 @@ impl Foo for () {
     fn bar(&self) -> usize { 3 }
 }
 
+// Test a case where MIR would inline the default trait method
+// instead of bailing out. Issue #40473.
 fn main() {
     let result = ().bar();
     assert_eq!(result, 3);